The present invention pertains to a process for manufacturing films by the blow extrusion process from semi-crystalline fluid polymers, consisting of coextruding the semi-crystalline fluid polymer with an incompatible thermoplastic resin.
Thin films of thermoplastic polymers are usually obtained by the blow extrusion process, consisting of extruding a thermoplastic polymer through an annular die, generally upward, with the extrudate being simultaneously pulled longitudinally by a drawing device, usually one with rollers, and inflated by a constant volume of air trapped between the die, the drawing system and the wall of the tube. The inflated tube, also called a bubble, is cooled as it emerges from the die, generally by an air blowing ring. The bubbles are flattened and rolled up either as a tube or; after being cut, into two separate films.
This conventional process is at least difficult if not impossible to use in the case of blow extrusion of semi-crystalline polymer tubes. In this case, the forces involved, such as the drawing force, internal pressure of the bubble, gravity and others, are applied to an extrudate whose mechanical strength is very low. This results in significant deformations of the bubble, leading to the formation of creases in the film after the bubble has been flattened and to a total absence of control over the thickness distribution of the film.